Yes, SSDs are really quite a bit better. You'll be best off to put all the cache, scratch, and previews on the 960 Pro boot drive which is by far the fastest drive. And I would think that 512GB would be enough to handle all these files in addition to your OS and programs.

OK, thanks. If I can have only one PCI-e based SSD in my system, would it make more sense to use a smaller SATA drive for the O/S and programs exclusively and increase the size of the NVME SSD, as follows:

Not really; it's more due to the fact that once the OS and programs have been loaded, they are largely RAM (memory) resident and don't need to read or write to the disk any more. Of course there is still some disk activity (on OS/programs drive), but nothing like the disk I/O going on with the bulk of the Premiere Pro files (media, scratch, cache, renders, etc.).

M.2 and PCIe drives are about 3 to 4 times faster (or even more) than a fast single SSD. So in your case I suggested putting most the files on your M.2 drive.

Personally, I do put my cache/scratch files on my boot drive, but it's a PCIe SSD. My data drive is faster for total throughput (8x SATA SSD array), however my PCIe drive (and M.2 drives) has very little overhead and work well with multiple small files.

Here's a pretty good link for the full details of Premiere Pro file, including scratch and media:

But really all you need to know is Premiere Pro creates and uses them for performance reasons and you can always delete them all and not loose anything that cannot be easily re-created. So, if you point them all to be stored in the same folder you know you can always get away with deleting them and you don't need worry about backing them up.